2 6.1 Light Sources An object is a source of light.A direct source produces light, e.g. the sun, light bulb, fire.An indirect source does not produce light, e.g. an illuminated object.An extended object may be regarded as a set of point sources.

7 (a) Black body : is an ideal absorber, also a perfect emitter6.1 Light Sources2. Blackbody Radiators(a) Black body : is an ideal absorber, also a perfect emitterA good way of making a blackbody is to force reflected light to make lots of reflections: inside a bottle with a small openingThe spectral distribution of that radiation is a function of temperature alone; the material as such plays no role

11 E = h • n Energy of radiation is proportional to frequency.where h = Planck’s constant = x J•sLight with large l (small n) has a small E.Light with a short l (large n) has a large E.(b) Photon: the oscillators emit energy, as discrete, elemental units of energy called quanta or photons

12 Photons Light also behaves as a stream of particles, called photons.Light has “wave-particle duality” , meaning that it behaves as waves and as particles.This is a concept in quantum mechanics.

13 (c) Black-body radiation is electromagnetic radiation that is in thermal equilibrium at a temperature T with matter that can absorb and emit without favouring any particular wavelength(d) Plank’s radiation law

20 4. Stefan-Boltzmann's Law6.1 Light Sources4. Stefan-Boltzmann's LawThe total energy density inside a blackbody cavity is given by integration over all wavelengthsNote that Intensity increases with TTemperature must be in Kelvin, where size of one Kelvin is same as size of one degree Celsius, but T=0K is absolute zero, and T=273K = 0oC (freezing).

21 5. Klrchhoff's Law 6.1 Light SourcesKirchhoff's law :an object that is a good radiator at a given wavelength is also a good absorber at the same wavelengthStefan-Boltzmann's law for gray bodiesfactor : the emissivity of the surfaceRecall that a good absorber is also a good emitter, and a poor absorber is a poor emitter. We use the symbol  to indicate the blackness ( =0) or the whiteness (=1) of an object.

22 ExampleIf you eat 2,000 calories per day, that is equivalent to about 100 joules per second or about 100 Watts - which must be emitted.Let’s see how much radiation you emit when the temperature is comfortable, say 75oF=24oC=297K, and pick a surface area, say 1.5m2, that is at a temperature of 93oF=34oC=307K:Memitted = AT4 =(5.67x10-8W/m2K4)*(.97)*(1.5m2)*(307K)4 = 733 Watts emitted!

24 6.2 Detectors thermal detectors based on absorption and heatingIf the absorbing material is black, they are independent of wavelength.quantum detectors.based on photoelectric effectQuantum detectors are of particular interest, both theoretical and practical; some of them are so sensitive they respond to individual quanta.

25 1. Thermal Detectors 6.2 Detectors slow to respond Golay cella thin black membrane placed over a small, gas-filled chamber. Heat absorbed by the membrane causes the gas to expand, which in turn can be measured, either optically (by a movable mirror) or electrically (by a change in capacitance).used in the infrared.

26 6.2 DetectorsThermocouplea junction between two dissimilar metals. As the junction is heated, the potential difference changes. In practice, two junctions are used in series, a hot junction exposed to the radiation, and a cold junction shielded from it. The two voltages are opposite to each other; thus the detector, which without this precaution would show the absolute temperature, now measures the temperature differential.thermopilecontains several thermocouples and, therefore, is more sensitive.

27 6.2 Detectorsbolometercontains a metal element whose electrical resistance changes as a function of temperature; if instead of the metal a semiconductor is used, it is called a thermistor.Unlike a thermocouple, a bolometer or thermistor does not generate a voltage; they must be connected to a voltage source.

28 the wavelength of the light plays an important role6.2 Detectors2. Quantum Detectorsthe wavelength of the light plays an important rolethere is a certain threshold above which there is no effect at all, no matter what the intensityintense light and dim light cause same of an effect

32 Photoelectric Effect (2)Classical theory said that E of ejectedelectron should increase with increasein light intensity — not observed!Experimental observations can be explained if light consists of particles called PHOTONS of discrete energy.

34 plate M(photocathode) 6.2 DetectorsVALighte-Variable powersupplyplate M(photocathode)when irradiated, releases electrons (called photoelectrons)collector plate C(anode)photoelectrons released by M are attracted by, and travel to C.As the potential V, read on an high-impedance voltmeter, is increased, the current, I, read on an ammeter, increases too, but only up to a given saturation level, because then all of the electrons emitted by M are collected by C.

35 6.2 Detectorsif C is made negative, some photocurrent will still exist, provided the electrons ejected from M have enough kinetic energy to overcome the repulsive field at C. But as C is made more negative, a point is reached where no electrons reach C and the current drops to zero. This occurs at the stopping potential, V0.In short: A significant amount of photocurrent is present only if the collector, C, is made positive

36 When the frequency of the light is increased, the stopping potential also increases.

37 The electron photo-current can be stopped by a retarding potentialThe electron photo-current can be stopped by a retarding potential. Increasing the light intensity do not change the retarding potential.

38 6.2 DetectorsIf more intense light falls on the photocathode, it will release more electrons but their energies, and their velocities, will remain the same.The energy of the photoelectrons depends on the frequency of the light: blue light produces more energetic photo-electrons than red light.The response of a quantum detector is all but instantaneous: there is no time lag, at least not more than 10-8 s, between the receipt of the irradiation and the resulting current.

39 Einstein's photoelectric-effect equation.6.2 DetectorsThe light is received in the form of discrete quanta.Part of the energy contained in a quantum is needed to make the electron escape from the surface; that part is called the work function, W.Only the excess energy, beyond the work function, appears as kinetic energy of the electron. The maximum kinetic energy with which the electron can escape, therefore, isKEmax = h - WEinstein's photoelectric-effect equation.

40 h = W + KEKE = h - WEinstein suggested that the linear behaviour is simply a Conservation of Energy.Energy of Light =Energy needed to get out +Kinetic Energy of electron.

41 Example - Photoelectric EffectGiven that aluminum has a work function of 4.08 eV, what are the threshold frequency and the cutoff wavelength?

42 6.2 DetectorsIt is often convenient to measure energies on an atomic scale not in joule but in electron volt, eV.1 eV = (1e)(1V) =  J

44 Photoelectric Properties Of Some Alkali Metals6.2 DetectorsThe work function determines the longest wavelength to which a detector can respond: the lower the work function, the longer the wavelength. The lowest work functions are found among the alkali metals.Photoelectric Properties Of Some Alkali MetalsAlkali Work function (eV) Threshold (nm)SodiumPotassiumRubidiumCesium

45 The Photoelectric Effect on PotassiumDetermine the work function WKE=(hc)(1/) － W

49 quantum efficiency:the ratio of the number of photoelectrons released to the number of photons received.Ordinarily, this efficiency is no higher than a few percent.Several diodes are combined in series to form a photomultiplier, the efficiency becomes much higher.Light strikes photocathode (-)Photocathode emits photoelectronsPhotoelectrons accelerate toward series of increasingly positive anodes (+) at which photoelectrons and secondary electrons are emitted (dynodes)Electrons accelerated toward collection anode

50 6.3 Practical Quantum DetectorsA photocell is the solid-state equivalent of the vacuum photodiode; most often it is a semiconductor.A semiconductor conducts electricity better than an insulator but not as well as a conductor.In an insulator, the electrons are tightly bound to their respective atoms.In a metal, the electrons can move freely; hence, even a small voltage applied to the conductor will cause a current.

51 6.3 Practical Quantum Detectorsphotoconductive detectors : semiconductor, such as cadmium sulfide (CdS), gallium arsenide, and silicon, conduct electricity poorly only in the dark; when exposed to light, they conduct very well.

52 photo-voltaic detectors:6.3 Practical Quantum Detectorsphoto-voltaic detectors:made from two semiconductors, one of them transparent to light, for instance a layer of CdS deposited on selenium. When light is incident on the junction, the electrons start moving, but only in one direction producing a current; in other words, the junction converts light energy into electrical energy.used as solar cells and as exposure meters in photographic cameras.

53 image tube:not only detects light but also preserves the spatial characteristics of an image.6.3 Practical Quantum Detectorscontain an array of photoconductors, one for each pixel. When exposed to light, the elements from a latent image that can be read by an electron beam scanning across them.the photoelectrons emitted by the cathode can be focused by an electron lens and made visible on a phosphor screen mounted in the same tube.

54 6.3 Practical Quantum Detectorsimage intensifier:the image is merely amplified.image converterthe image is formed in the IR, the UV or the X-ray range and converted into the visiblemicrochannel image intensifierthe system is built around an array of many short fibers or capillaries, fused into a wafer.